Biological production of acetic acid from waste gases with Clostridium ljungdahlii

ABSTRACT

A method and apparatus for converting waste gases from industrial processes such as oil refining, carbon black, coke, ammonia, and methanol production, into useful products. The method includes introducing the waste gases into a bioreactor where they are fermented to various organic acids or alcohols by anaerobic bacteria within the bioreactor. These valuable end products are then recovered, separated and purified. In an exemplary recovery process, the bioreactor raffinate is passed through an extraction chamber into which one or more non-inhibitory solvents are simultaneously introduced to extract the product. Then, the product is separated from the solvent by distillation. Gas conversion rates can be maximized by use of centrifuges, hollow fiber membranes, or other means of ultrafiltration to return entrained anaerobic bacteria from the bioreactor raffinate to the bioreactor itself, thus insuring the highest possible cell concentration.

The U.S. Government has license rights in this invention pursuant toD.O.E. Cooperative Agreement No. DE-FC02-90CE40939 awarded by the U.S.Department of Energy.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 07/968,857,filed Oct. 30, 1992, now abandoned.

BACKGROUND OF THE INVENTION

The present invention is directed to biological methods, processes andapparatus for producing organic acids, alcohols, and salts from thewaste gas streams of certain industrial processes and more particularlyconcerns a process utilizing continuous gaseous substrate fermentationunder anaerobic conditions to accomplish this conversion.

The conventional procedure for producing organic acids, alcohols, andsalts is chemical synthesis of petroleum-derived feedstocks. The rapidlyescalating cost of petroleum has generated considerable interest inproducing these valuable commodities by fermentative processes thatutilize renewable or waste materials as the feedstock.

There is also growing concern over the massive amounts of atmosphericpollutants and greenhouse gases produced by conventional industrialprocesses. The Environmental Protection Agency recently estimated thatover six million metric tons of carbon monoxide and nearly four millionmetric tons of hydrogen were discharged annually by the industrialcomplex. A substantial portion of this waste carbon monoxide andhydrogen are the result of carbon black manufacture and coke production,roughly 2.6 million metric tons of CO and 0.5 million metric tons of H₂.Large amounts of carbon monoxide or hydrogen are also produced by theammonia industry (125,144 metric tons of CO in 1991), petroleum refining(8 metric tons per thousand barrels), steel mills (152 pounds per metricton of steel produced), and sulfate pulping of wood (286 pounds per tonof pulp). In 1991, the adipic acid industry generated 40,773 metric tonsof carbon monoxide that was burned for fuel value or flared. In manycases, these gases are discharged directly to the atmosphere, placing aheavy pollution burden on the environment.

Typically, the waste gases from the manufacture of industrial productsare released at low pressures and temperatures. Current technology cannot utilize these dilute gases under such conditions. Adapting existingtechnology to separate and recover hydrogen or carbon monoxide fromthese waste streams would be expensive and impractical.

In light of the foregoing, there is a need for a cost effective andpractical method and apparatus for utilizing the above-described wastegases and for producing organic acids, alcohols and salts by other thanchemical synthesis of petroleum derived feedstocks.

SUMMARY OF THE INVENTION

In accordance with the present invention, organic acids, alcohols,and/or salts are produced from the waste carbon monoxide, hydrogen,and/or carbon dioxide of industrial processes, thereby reducingenvironmental pollution while at the same time saving energy andchemical feedstocks.

In accordance with an exemplary process of the present invention, thedesired components of the dilute gas mixtures are introduced into abioreactor containing one or more cultured strains of anaerobic bacteriathat utilize the waste gas components by a direct pathway to produce adesired organic compound. The organic compound is recovered from theaqueous phase in a separate vessel or vessels, utilizing a suitablerecovery process for the compound produced. Examples of recoveryprocesses include extraction, distillation or combinations thereof, orother efficient recovery processes. The bacteria are removed from theaqueous phase and recycled to avoid toxicity and maintain high cellconcentrations, thus maximizing reaction rates. Cell separation isaccomplished by centrifugation, membranous ultrafiltration, or othertechniques.

Although not limited thereto, the methods and apparatus of the presentinvention are especially adapted to the production of acetic acid andethanol from a waste gas stream of identical composition to that foundin the manufacture of carbon black.

The principal object of the present invention is the provision of aprocess for the production of organic acids, alcohols, and/or salts fromcarbon monoxide, hydrogen, and/or carbon dioxide.

Another object of the present invention is the provision of methods andapparatus for the production of organic acids, alcohols, and/or saltsfrom the waste gas streams of industrial processes such as oil refining,carbon black, coke, ammonia, and methanol production.

A still further object of the present invention is the provision of aprocess for producing acetic acid and ethanol from a waste gas stream ofidentical composition to that found in the manufacture of carbon black.

Yet another and more particular object of the present invention is theprovision of a method and apparatus involving continuous gaseoussubstrate fermentation under anaerobic conditions to accomplish theconversion of waste gas streams of certain industrial processes intouseful products such as organic acids, alcohols, and salts.

Other objects and further scope of the applicability of the presentinvention will become apparent from the detailed description to follow,taken into conjunction with the accompanying drawings wherein like partsare designated by like reference numerals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the method and apparatus in accordancewith an exemplary embodiment of the present invention;

FIG. 2 is a schematic representation of a continuous fermentation systemin accordance with one embodiment of the present invention;

FIG. 3 is a graphical illustration of cell concentration (OD) versustime; and

FIG. 4 is a graphical representation of acetic acid concentration (HAC)versus time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The term "waste gas" or "waste gas stream" as used herein means carbonmonoxide and hydrogen mixed with other elements or compounds, includingcarbon dioxide, in a gaseous state and which are typically released orexhausted to the atmosphere either directly or through combustion.Normally, release takes place under standard smokestack temperatures andpressures. Accordingly, the processes of the present invention aresuitable for converting these atmospheric pollutants into usefulproducts such as organic acids, alcohols and salts. These productsinclude, but are not limited to acetic, propionic, and butyric acids,methanol, ethanol, propanol, and n-butanol, plus salts, such as calciummagnesium acetate.

Anaerobic bacteria which convert carbon monoxide and water or hydrogenand carbon dioxide into alcohols and acids include Acetobacterium kivui,A. woodii, Butyribacterium methylotrophicum, Clostridium aceticum, C.acetobutylicum, C. formicoaceticum, C. kluyveri, C. thermoaceticum, C.thermocellum, C. thermohydrosulfuricum, C. thermosaccharolyticum,Eubacterium limosum, and Peptostreptococcus productus. In thedevelopment of the present invention, new strains of anaerobic bacteriahave been isolated which enact this conversion with high efficiency.Depending on the specific microorganism(s) utilized, variables whichmust be considered include nutrient levels in the nutrient medium,pressure, temperature, gas flow rate, liquid flow rate, reaction pH,agitation rate (if utilizing a Continuously Stirred Reactor), inoculumlevel, maximum substrate (introduced gas) concentrations to avoidinhibition, and maximum product concentrations to avoid inhibition.

In accordance with an exemplary embodiment of the present invention andas shown in FIG. 1 of the drawings, a first step in the conversionprocess is the utilization of a mixing tank 10 to prepare the nutrientmedia for the anaerobic bacteria. The content of the nutrient media willvary based on the type of anaerobe utilized. The nutrients areconstantly fed to a bioreactor 12 (fermenter), consisting of one or morevessels and/or towers of a type which includes the Continuously Stirred(CSTR), Immobilized Cell (ICR), Trickle Be(I (TBR), Bubble Column, AirLift Fermentors, or other suitable fermentation reactor. Within thebioreactor 12 resides the culture, either single or mixed species, ofanaerobic bacteria utilized in the gas conversion process. For theCSTRs, these bacteria live dispersed throughout the media, but for ICRs,the bacteria adhere to an internal packing medium. This packing mediummust provide maximal surface area, high mass transfer rate, low pressuredrop, even gas and liquid distribution, and must minimize plugging,fouling, nesting and wall channeling. Examples of such medium materialsare ceramic berl saddles or other high performance packings.

Next, the waste gases are introduced into the bioreactor. The gas isretained in the bioreactor for the period of time which maximizesefficiency of the process. Exhaust gases, if any, are then released. Theliquid effluent or broth 14 is passed to a centrifuge, hollow fibermembrane, or other filtration device 16 to separate out microorganismsthat are entrained. These microorganisms 18 are returned to thebioreactor to maintain a high cell concentration which yields a fasterreaction rate.

A next step in the process is separation of the desired biologicallyproduced product(s) from the permeate or centrifugate 20. For example,the permeate or centrifugate 20 is passed to an extraction chamber 22where it is contacted with a solvent 24. The solvent 24 should have ahigh distribution coefficient for the desired end product, a highrecovery factor, low toxicity to humans, low toxicity to the bacteria,immiscibility with water, appropriately high boiling point, and form noemulsion with the bioreactor constituents. The distribution of solutebetween solvent and aqueous phase will determine the thermodynamicfeasibility and the amount of solvent required to remove the endproduct. Typical solvents include tributyl phosphate, ethyl acetate,tri-octyl phosphine oxide, long chain alcohols, hexane, cyclohexane,chloroform, and tetrachloroethylene.

The nutrients and materials in the aqueous phase 26 pass back to thebioreactor and the solvent/acid/water or solvent/alcohol/water solution28 passes to a distillation column 30, where it is heated to asufficient temperature to separate the solvent from the alcohol or acidand water. The solvent 32 passes from the distillation column through acooling chamber 34 to lower the temperature to the optimum temperaturefor extraction, then back to the extraction chamber 22 for reuse. Thealcohol or acid and water solution 36 passes to a final distillationcolumn 38 where the desired end product 40 is separated from the waterand removed. The water 42 is recirculated to the mixing tank fornutrient preparation. With reference again to FIG. 1, the systemincludes a plurality of conventional heat exchangers HE in the form ofcondensers (arrow pointing downwardly) and reboilers (arrow pointingupwardly). Also, cross exchangers CE are located at the junctures oflines 20 and 26 and lines 24 and 28.

Thus in accordance with the present invention it is now possible toproduce valuable organic acids, alcohols, or salts by a gaseoussubstrate fermentation, not only reducing consumption of valuablechemical feedstocks, but also removing hazardous atmospheric pollutantsfrom the waste gas streams of many industries. Previous processes toderive these chemicals biologically were based on fermentation ofsugars.

The following specific examples are submitted to illustrate but not tolimit the present invention. Unless otherwise indicated, all parts andpercentages in the specification and claims are based upon volume.

EXAMPLE 1 Production of Acetic Acid from Carbon Black Waste Gases

This example is directed to a process utilized to convert waste gas of acomposition which matches that of the furnace exhaust of carbon blackmanufacture to acetic acid. The waste gas has a composition of about 13percent carbon monoxide, 14 percent hydrogen, and 5 percent carbondioxide, with the remaining 68 percent largely nitrogen with traces ofoxygen and sulfur compounds. The waste gases are produced as the resultof partial oxidation of gas or oil with insufficient air to formamorphous carbon, with about 1.2 pounds of carbon monoxide produced perpound of elemental carbon. These waste gases form a serious atmosphericcontamination problem and also represent a valuable chemical feedstockresource not presently being recovered.

Microbiology of Acetic Acid Production

Bioconversion of the gases H₂, CO, and CO₂, by anaerobic bacteria hasbeen known and has been demonstrated in accordance with the presentinvention as having economic potential (Barik et al., 1985, 1986; 1987;Vega et al., 1988; 1989a; Clausen and Gaddy, 1985; Klasson et al., 1990;Gaddy and Clausen, 1987). Several bacterial species are capable ofconverting these gases into acetate, which is an intermediate product inmany biological pathways.

Bacterial species such as Acetogenium kivui (Leigh et al., 1981),Peptostreptococcus productus (Barik et al., 1986, Lorowitz and Bryant,1984), Acetobacterium woodii (Kerby et al., 1983), Clostridiumthermoaceticum (Wood et al., 1982, Kerby and Zeikus, 1983), andEubacterium limosum (Genthner and Bryant, 1982) produce acetate by thereaction: ##STR1##

Many anaerobic bacteria are also known to produce acetic acid from H₂and CO₂ (Mayer et al., 1977; Sleat et al., 1983; 1985; Balch et al.,1977). These Bacterial isolates include A. kivui, P. productus, andAcetobacterium sp. (Balch et al., 1977), which utilize homoaceticfermentation by anaerobically oxidizing hydrogen and CO₂ according tothe equation: ##STR2##

Acetobacterium woodii (Mayer et al., 1977) and Acetoanaerobium noteraeproduce acetate from H₂ and CO₂ according to the above reaction, but inaddition to acetate, A. noterae produces some propionate and butyrate.Another chemolithotrophic bacteria, Clostridium aceticum, producesacetate from CO₂ using a glycine decarboxylase pathway (Waber and Wood,1979).

Some bacteria, like A. kivui, P. productus, and A. woodii, produceacetate from either CO and H₂ O or H₂ and CO₂ (Vega et al., 1989; Bariket al., 1986). P. productus gives particularly fast rates of conversionand demonstrates high tolerance to CO; however, this organism shows apreference to follow Equation (1) over Equation (2).

In addition to these listed bacteria, two strains of an additionalClostridia which produce acetic acid or ethanol from CO and H₂ O or H₂and CO₂ have been isolated. One is a Clostridium ljungdahlii ERI2rod-shaped, gram positive, non-thermophilic anaerobe which givessuperior acetic acid yields and operates at a low pH, which greatlyenhances the recovery of the product. C. ljungdahlii ERI2 carries out avigorous acetogenic fermentation of glucose. It also infrequently formsspores and carries out a primarily acetogenic fermentation of hexose orH₂ :CO₂. It is motile with peritrichous flagellation. This new strain ofC. ljungdahlii, referred to as ERI2, was isolated from a natural watersource and was deposited on or about Dec. 8, 1992 with the American TypeCulture Collection (ATCC), 12301 Parklawn Drive, Rockville, Md. 20852,U.S.A. accession no. 55380. The other, C. ljungdahlii PETC, is describedin allowed U.S. patent application Ser. No. 07/612,221 filed on Nov. 9,1990, now U.S. Pat. No. 5,173,429 issued on Dec. 22, 1992, and isdeposited with the American Type Culture Collection (ATCC), Rockville,Md., accession no. 49587.

In the development of the present process, two distinct routs to produceacetic acid from carbon black waste gases were studied. The direct routeconverts Co and H₂ O or H₂ and CO₂ directly into acetic acid accordingto Equations (1) and (2), respectively. An indirect route involves theconversion of CO and H₂ O into H₂ and CO₂ by the water gas shiftreaction, followed by production of acetic acid from H₂ and CO₂. Thisindirect route was found to be a less efficient utilization of thetechnology.

The acetogens tested are summarized in Table 1. Among these bacteriathat produce acetic acid directly from Co, A. kivui and the newlyisolated strain, C. ljungdahlii ERI2, show far superior rates for bothCo and H₂ utilization. Further experimentation proceeded using these twoanaerobic bacteria.

There are obvious advantages to the bacteria utilizing carbon monoxideand hydrogen simultaneously. This would afford the most efficient use ofthe waste gases and remove the greatest amount of atmosphericpollutants.

Bench Scale Operation of the Described Process to Produce Acetic Acid

As shown in FIG. 2 of the drawings and in accordance with one embodimentof the present invention, a bench scale continuous conversion system isshown to include a BioFlo IIC fermentor 50 from New Brunswick ScientificCo., Inc., Edison, N.J. The fermentor 50 is equipped with an agitationmotor, pH controller, foam controller, thermostat, dissolved oxygenprobe, nutrient pump, and 2.5 L culture vessel. The working volume isvariable (1.5-2.0 L). Other variable operational parameters includemedium feeding rate (Dilution rate), gas flow rate (Gas retention time),agitation (rpm). The vented or exhaust gases exit the fermentor 50through a condenser fixed to a vented hood via a water trap and asampling port.

The culture broth 52 is recycled through a cross-flow hollow fibermodule 54 by a peristaltic pump (from Cole Parmer). The recycling rateis about 80-100 mL/min. The hollow fiber module 54 has the followingcharacteristics; the surface area is 0.35 ft², the pore size is 0.2 uand the lumen diameter is 1 mm. The permeate 56 is pumped to a storagetank 58 (Feed storage). The culture cells are returned to the fermenteralong line 55.

A counter-current acetic acid extraction system, including two stagemixer and settler components includes first and second mixers 60 and 62and first and second settling tanks 64 and 66. The permeate 68 fromstorage 58 is pumped to mixer 60 through a flow controller 70. Thesolvent 72 is pumped to mixer 62 from solvent storage 74 through a flowcontroller 76. Both mixer 60 and mixer 62 are equipped with a stirringmechanism to achieve good mixing of aqueous phase and solvent phase. Themixture of both phases from the mixers 60 and 62 is led to settlers 64and 66, respectively. The phase separation is accomplished in thesettlers. The aqueous phase 78 from settler 64 is pumped to mixer 62,the solvent phase 80 from settler 64 is pumped to a separator 82, theaqueous phase 84 form settler 66 is pumped to raffinate storage 86, andthe solvent phase 88 from settler 66 is pumped to mixer 60. Theraffinate is recycled to the CSTR 50 along a line 90. This recycle line90 is partially bled at 92 to remove inhibiting factors.

The solvent 80 Loaded with acetic acid is pumped to a distillation flask94 through a preheater 96. The distillation flask 94 is equipped withtwo thermocouples 98 and 99 to monitor and control temperature in theliquid phase and gas phase. The heating temperature for distillation isset to achieve maximum vaporization of the acetic acid. The acetic acidvapors are condensed in a condenser 100 and collected in a flask 102.The stripped solvent 104 is pumped through a cooling coil 106 to solventstorage 74. With reference again to FIG. 2, the system includesconventional pumps P1-P14, sample ports S1-S12, T-intersections T1-T8,pressure gauges G1-G3, flow meters F1-F3, and valves V1, V2, and N1.

A bench scale operation of the described process as diagrammed in FIG. 2was fabricated in the laboratory to determine quantitative yields underoptimized conditions. The nutrient mixture fed to the culture was asfollows:

1. 80.0 ml of a salt, composed of

    ______________________________________                                               KH.sub.2 PO.sub.4                                                                            3.00   g/L                                                     K.sub.2 HPO.sub.4                                                                            3.00   g/L                                                     (NH.sub.4).sub.2 SO.sub.4                                                                    6.00   g/L                                                     NaCl           6.00   g/L                                                     MgSO.sub.4.2H.sub.2 O                                                                        1.25   g/L                                              ______________________________________                                    

2. 1.0 g of yeast extract

3. 1.0 g of trypticase

4. 3.0 ml of PFN (Pfenning) trace metal solution

    ______________________________________                                        FeCl.sub.2 *4H.sub.2 O                                                                             1500   mg                                                ZnSO.sub.4 *7H.sub.2 O                                                                             100    mg                                                MnCL.sub.2 *4H.sub.2 O                                                                             30     mg                                                H.sub.3 BO.sub.3     300    mg                                                CoCl.sub.2 *6H.sub.2 O                                                                             200    mg                                                CuCl.sub.2 *H.sub.2 O                                                                              10     mg                                                NiCl.sub.2 *6H.sub.2 O                                                                             20     mg                                                NaMoO.sub.4 *2H.sub.2 O                                                                            30     mg                                                Na.sub.2 SeO.sub.3   10     mg                                                Distilled water      1000   ml                                                ______________________________________                                    

5. 10.0 ml of B vitamins

    ______________________________________                                        Pyridoxal HCl          10     mg                                              Riboflavin             50     mg                                              Thiamine HCl           50     mg                                              Nicotinic acid         50     mg                                              Ca-D-Pantotheinate     50     mg                                              Lipoic Acid            60     mg                                              P-aminobenzoic acid    50     mg                                              Folic acid             20     mg                                              Biotin                 20     mg                                              Cyanocobalamin         50     mg                                              Distilled water        1000   ml                                              ______________________________________                                    

6. 0.5 g of Cysteine HCL

7. 0.06 g of CaCl₂ 2H₂ O

8. 2.0 g of NaHCO₃

9. 1.0 ml of Resazurin (0.01%)

10. 920.0 ml of distilled water

For use with A. kivui, the nutrient solution was pH adjusted to 6.6,whereas for the new strain, C. ljungdahlii ERI2, the pH was adjusted to4.9. As will be pointed out, the ability to operate at a lower pH is agreat advantage in acetic acid recovery. The solution was then spargedfor 20 minutes with a 20% CO₂ and 80% N₂ atmosphere, then transferredanaerobically and autoclaved for 15 minutes.

Numerous experiments were carried out with both Continuous StirredReactors (CSTR) and Immobilized Cell Reactors (ICR). The resultsobtained are exemplified in the following data.

CSTR Experiments Utilizing the Bacterial Strains A. kivui and C.ljungdahlii ERI2

The bench scale system operating with the CSTR and the anaerobicbacteria, C. ljungdahlii ERI2 and A. kivui, consisted of a New BrunswickScientific Bioflow IIc fermenter, a hollow fiber membrane unit for cellrecycle, and extraction and distillation columns. Nutrient mixture wasfed into the bioreactor at a rate of 3.2 cubic centimeters per minute.Capacity of the reactor was 2.5 liters, within which a constant fluidlevel of 1.5 liters was maintained. The fluid was agitated at variablerates of up to 1000 revolutions per minute with gas introduced at a rateof approximately 500 cubic centimeters per minute. Optimal gas retentiontimes were in the range of three minutes. The gas feed varied with itsuptake by the bacteria, which was in turn a function of the celldensity.

The liquid from the bioreactor was passed to the hollow fiber membraneat a rate of 55 to 70 milliliters per minute. From the hollow fibermembrane, permeate was gathered at a rate of 1.5 milliliters per minute.Analysis of this permeate indicates the acetic acid/acetateconcentration at this stage to range in excess of 20 grams per liter.Operating at a pH of 4.9, 42 percent of this product was in the acidform using C. ljungdahlii ERI2. For A. kivui, the acid yield was only1.4 percent. Results of various runs for the two bacteria, includingconversion rates and product yields are summarized in Tables 2 and 3.

ICR Experiments Utilizing the Bacterial Strain C. ljungdahlii ERI2

An Immobilized Cell Reactor (ICR), consisting of a 2 inch outsidediameter by 24 inch tall glass tube packed with fabric to support thecells and Enkamat 7020 as an immobilizing medium was also tested in theacetic acid production process. With C. ljungdahlii ERI2 as theacetogenic anaerobe, 100 percent of the carbon monoxide and 79 percentof the hydrogen were converted at a gas retention time of 20 minutes.Acetic acid concentrations in the removed liquid were approximately 6.0grams per liter. Results of the ICR studies are summarizes in Table 4.

The ICR has a certain attractiveness on an industrial scale in that theenergy costs to operate the reactor are reduced significantly. Theproper selection of packing materials, solution phases, and pressuresmay yield production approaching that of the CSTR.

Acetic Acid Recovery

Various solvents were tested for recovering acetic acid from thepermeate, the results are summarized in Table 5. Tributyl phosphate wasidentified as having both a high distribution coefficient and a highboiling point. The solvent and permeate from the cell separator werecommingled in a two stage extraction process. Alternatively, anextraction column could be used. Permeate was introduced into a 3 literflask where it was mixed with incoming solvent. A ratio of 1 partsolvent to 1 part permeate worked well and gave high recovery rates. Thecombined fluids were passed from the mixer to a 4 liter settling chamberwhere the solvent/acetic acid mixture separate as a lower density phasefrom the water and nutrients. Retention times of approximately 15minutes were used in the settling tanks. The lower density phase wasextracted and fed to a distillation flask. The raffinate was passed fromthe first settler to a second mixer where it was contacted again withsolvent, then removed to a second settling chamber. This allowed formore complete extraction of the acetic acid; acid recovery increasedfrom 82 percent to greater than 96 percent using tributyl phosphate. Thesolvent/acetic acid mixture from this settler was returned to the firstmixer, while the raffinate of water and organics was passed back to thebioreactor.

The distillation unit was a 5 liter flask with a boiling mantle. Acommon distillation column, with reflux, could be used for complete acidrecovery. Because of the high boiling point of tributyl phosphate,nearly complete recovery is accomplished in one step. The solvent/aceticacid mixture was heated to 120 degrees C., with the acetic acidcollected overhead in a condensing coil. In this single stage system,distillation efficiencies of 70 percent were achieved.

Solvent mixtures were also tried and distribution coefficients of mixedsolvents are summarized in Table 6.

EXAMPLE 2 Production of Ethanol from Carbon Black Waste Gases

Microbiology of Ethanol Production

Bioconversion of carbon monoxide, hydrogen, and carbon dioxide toethanol has received relatively little study, hence not many organismswhich carry out this reaction are known. This is in part a result of thepreference of most anaerobic microorganisms utilizing the necessarypathway to produce acetate. At the present time, only C. ljungdahliiPETC (ATCC No. 49587) and ERI2 (ATCC No. 55380) are known to produceethanol in significant amounts from gaseous substrates containing carbonmonoxide. This reaction takes place according to the followingstoichiometry:

    6CO+3H.sub.2 O→CH.sub.3 CH.sub.2 OH+4CO.sub.2       (3)

    2CO.sub.2 +6H.sub.2 →CH.sub.3 CH.sub.2 OH+3H.sub.2 O (4)

Bench Scale Process to Produce Ethanol

The bench scale process to produce ethanol was essentially the same asthat for producing acetic acid. Ethanol production takes place at theexpense of acetic acid production in this system. Variables whichenhance ethanol yield include: nutrient limiting the culture; addingbiotin and thiamine to the culture; lowering pH and dilution rates; andusing reducing agents, such as cysteine hydrochloride or benzyl viologenin low concentrations. Productivity is maximized by assuring thatadequate amounts of gaseous substrate are available to the culture(ethanol is consumed when the gas is limited) and by maintaining highagitation rates when using the CSTR. Continuous product recovery, usingextractive fermentation is also essential to prevent conversion ofethanol to acetate.

Operating with a feed batch unit, C. ljungdahlii ERI2 performed asfollows:

pH: 4.5

Agitation Rate: 1000 RPM

Gas Flow Rate: 20 ml/min

CO Conversion Rate: 65-75%

Ethanol Production: 9.7 g/L

Acetic Acid Production: 2.0 g/L

EXAMPLE 3 Production of Acetic Acid from Carbon Black Waste Gases atHigher Pressures

Mass transport in the cellular reactions can be further enhanced byoperating the system at increased pressures. Simple batch experimentswere carried out to test the dynamics of this system. It was found thatreaction rates increased in linear proportion to the pressure, with acorresponding reduction in effective retention time. Another advantageto operating at increased pressure is that reactor volume can also bereduced in linear fashion, i.e. operation at 10 atmospheres pressurerequires a reactor with one tenth the volume of a reactor operating at 1atmosphere. FIGS. 2 and 3 show the increase in cell density and aceticacid concentration, respectively, with the increased pressure. Thisacetic acid concentration far exceeds typical batch concentrations for abatch reactor at atmospheric pressure.

EXAMPLE 4 Production of Acetic Acid from Carbon Black Waste Gases withSurfactants

Mass transport is also increased by the use of surfactants. Table 7presents the results of carbon monoxide uptake tests performed on C.ljungdahlii ERI2 in the presence of various commercial surfactants. Ineach case, the control value of 100 (percent) represents CO uptake inbatch fermentation, and the sample value, the percentage of the controlin batch fermentation in the presence of the surfactant.

In accordance with one embodiment, the present invention is directed toa process for producing organic acids, alcohols, or salts from a wastegas stream, including the steps of: fermenting the waste gasanaerobically with at least one microorganism in a bioreactor to producean organic acid, alcohol, or salt product, continuously removing aproduct containing broth from the bioreactor, and recovering the productacid, alcohol, or salt from the broth of the bioreactor. The waste gasis generated by industrial processes such as carbon black, ammonia, ormethanol production, petroleum refining, or coke manufacture andcontains carbon monoxide and water and/or carbon dioxide and hydrogen.The microorganism is of a species which produce organic acids oralcohols by anaerobic fermentation of gaseous substrates. The productsare selected from the group including acetic acid, propionic acid,butyric acid, methanol, ethanol, propanol, N-butanol, and salts, such ascalcium magnesium acetate.

Thus it will be appreciated that as a result of the present invention, ahighly effective improved process for converting waste gases to organicacids or alcohols is provided by which the principal objective, amongothers, is completely fulfilled. It is contemplated, and will beapparent to those skilled in the art from the preceding description andaccompanying drawings, that modifications and/or changes may be made inthe illustrated embodiments without departure from the presentinvention. Accordingly, it is expressly intended that the foregoingdescription and accompanying drawings are illustrative of preferredembodiments only, not limiting, and that the true spirit and scope ofthe present invention be determined by reference to the appended claims.

                  TABLE 1                                                         ______________________________________                                        ACETOGENIC BACTERIA TESTED FOR CO, H.sub.2,                                   AND CO.sub.2 CONVERSION                                                       Bacterial Route    Simultaneous Consumption                                   Direct Route       of CO and H.sub.2                                          ______________________________________                                        P. productus       No                                                         E. limosum         No                                                         A. noterae         No                                                         C. aceticum        No                                                         C. thermoaceticum  No                                                         S. sphaeroides     No                                                         A. woodii          Yes                                                        A. kivui           Yes                                                        C. ljungdahlii ERI2                                                                              Yes                                                        Indirect Route                                                                R. rubrum (ATCC 9791)                                                                            No                                                         R. rubrum (ATCC 25903)                                                                           No                                                         R. gelatinosa      No                                                         ______________________________________                                    

                                      TABLE 2                                     __________________________________________________________________________    Summary of ERI2 Experiments in the CSTR with Cell Recycle                     Gas Liquid  Percent                                                                             Dry Cell                                                                           Product                                                Reten.                                                                            Dilution                                                                          Agitat.                                                                           Gas   Weight                                                                             Concentration                                                                        Specific                                        Time                                                                              Rate                                                                              Rate                                                                              Conversion                                                                          Conc.                                                                              HAC                                                                              ETOH                                                                              Productivities                                  (min)                                                                             (hr.sup.-1)                                                                       (rpm)                                                                             CO H.sub.2                                                                          (g/L)                                                                              (g/L)                                                                            (g/L)                                                                             (g/L hr)                                                                          (g/g hr)                                    __________________________________________________________________________    9.30                                                                              0.056                                                                              750                                                                              80.75                                                                            74.5                                                                             2.3  9.7                                                                              0.07                                                                              0.43                                                                              0.18                                        9.28                                                                              0.055                                                                              750                                                                              82.1                                                                             72.0                                                                             3.32 9.56                                                                             0.094                                                                             0.52                                                                              0.16                                        6.14                                                                              0.061                                                                              750                                                                              73.6                                                                             46.5                                                                             4.11 12.78                                                                            0.125                                                                             0.78                                                                              0.19                                        6.4 0.08                                                                               750                                                                              74.8                                                                             49.6                                                                             5.02 12.98                                                                            0.125                                                                             1.05                                                                              0.19                                        4.74                                                                              0.087                                                                              750                                                                              68.5                                                                             37.2                                                                             4.79 12.38                                                                            0.125                                                                             1.08                                                                              0.23                                        4.91                                                                              0.10                                                                               750                                                                              68.8                                                                             50.2                                                                             4.53 10.73                                                                            0.05                                                                              1.08                                                                              0.24                                        4.05                                                                              0.102                                                                              750                                                                              65.5                                                                             58.1                                                                             5.27 11.49                                                                            0.076                                                                             1.17                                                                              0.22                                        3.98                                                                              0.103                                                                              900                                                                              74.3                                                                             67.9                                                                             6.17 12.73                                                                            0.1 1.31                                                                              0.21                                        2.89                                                                              0.117                                                                              900                                                                              66.1                                                                             33.9                                                                             5.91 11.69                                                                            0.04                                                                              1.38                                                                              0.23                                        3.28                                                                              0.105                                                                             1000                                                                              74.6                                                                             51.3                                                                             7.30 12.83                                                                            0.13                                                                              1.35                                                                              0.18                                        3.22                                                                              0.125                                                                             1000                                                                              73.1                                                                             54.0                                                                             10.25                                                                              13.57                                                                            0.08                                                                              1.71                                                                              0.17                                        2.65                                                                              0.13                                                                              1000                                                                              68.9                                                                             44.0                                                                             11.0 14.63                                                                            0.12                                                                              1.90                                                                              0.17                                        2.3 0.134                                                                             1000                                                                              66.0                                                                             38.7                                                                             11.1 20.59                                                                            0.113                                                                             2.77                                                                              0.25                                        2.7 0.11                                                                              1000                                                                              72.7                                                                             67.7                                                                             8.37 25.62                                                                            0.27                                                                              2.88                                                                              0.34                                        2.4 0.11                                                                              1000                                                                              68.6                                                                             63.3                                                                             9.88 26.82                                                                            0.36                                                                              2.95                                                                              0.30                                        2.55                                                                              0.122                                                                             1000                                                                              72.1                                                                             67.4                                                                             9.82 25.62                                                                            0.72                                                                              3.12                                                                              0.32                                        3.0 0.13                                                                              1000                                                                              76.6                                                                             73.3                                                                             12.4 22.33                                                                            0.52                                                                              2.90                                                                              0.23                                        __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________    Summary of A. kivui Experiments in the CSTR with Cell Recycle                 Gas Liquid  Percent Dry Cell                                                  Reten.                                                                            Dilution                                                                          Agitat.                                                                           Gas     Weight                                                                             Product                                                                           Specific                                         Time                                                                              Rate                                                                              Rate                                                                              Conversion                                                                            Conc.                                                                              Concen.                                                                           Productivities                                   (min)                                                                             (hr.sup.-1)                                                                       (rpm)                                                                             CO  H.sub.2                                                                           (g/L)                                                                              (g/L)                                                                             (g/L hr)                                                                          (g/g hr)                                     __________________________________________________________________________    5.0 0.058                                                                             750 67.8                                                                              44.2                                                                              4.00 16.15                                                                             0.96                                                                              0.24                                         4.4 0.958                                                                             750 65.7                                                                              38.5                                                                              4.8  16.63                                                                             0.94                                                                              0.19                                         4.3 0.058                                                                             900 71.3                                                                              40.7                                                                              4.5  17.03                                                                             0.99                                                                              0.21                                         3.72                                                                              0.058                                                                             900 69.0                                                                              37.3                                                                              5.14 19.16                                                                             1.13                                                                              0.22                                         3.72                                                                              0.076                                                                             900 70.3                                                                              41.1                                                                              5.28 16.17                                                                             1.21                                                                              0.23                                         3.2 0.076                                                                             900 66.4                                                                              41.4                                                                              5.71 16.85                                                                             1.23                                                                              0.23                                         2.8 0.076                                                                             900 61.5                                                                              29.1                                                                              5.00 16.16                                                                             1.22                                                                              0.23                                         2.8 0.076                                                                             1000                                                                              69.5                                                                              36.3                                                                              5.8  18.58                                                                             1.62                                                                              0.29                                         2.8 0.11                                                                              1000                                                                              70.2                                                                              41.6                                                                              5.9  18.4                                                                              1.84                                                                              0.36                                         2.2 0.11                                                                              1000                                                                              64.0                                                                              28.0                                                                              7.2  16.5                                                                              2.1 0.3                                          __________________________________________________________________________

                  TABLE 4                                                         ______________________________________                                        Fabric ICR Performance with ERI2                                              Liquid                                                                              Gas                CO          Product                                  Dilution                                                                            Retention                                                                              H.sub.2   Con-  Cell  Concentration                            Rate  Time     Conversion                                                                              version                                                                             Concen.                                                                             HAC   ETOH                               (hr)  (min)    (%)       (%)   (g/L) (g/L) (g/L)                              ______________________________________                                        0.23  4.83     38.62     54.66 .125  3.221 .778                                     7.41     49.15     70.87 .120  2.690 .620                                     11.66    51.31     80.61 .067                                                 13.61    56.87     83.93 .064  2.099 .201                               0.17  6.39     48.15     73.27 .161  3.382 1.366                                    11.21    68.96     92.82 .143  3.189 .495                                     55.44    83.13     98.27 .112  .813  .058                               0.12  6.26     43.89     70.76 .094  3.864 1.689                              0.09  7.87     42.40     79.72 .095  4.423 2.733                                    19.82    59.63     92.92 .102                                           0.03  22.14    55.01     94.21 .071  4.878 2.631                                    29.00    78.60     100   .018  5.604 2.748                                    60.48    83.33     100                                                  ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                        Acetic Acid Distribution Coefficient Study                                                   Equilibrium Aqueous                                                                         Acetic Acid                                                     Acetic Acid   Distribution                                     Solvent        Concentration, g/L                                                                          Coefficients                                     ______________________________________                                        Hexane         6.559         0.0                                              Decane         5.968         0.08                                             Chloroform     5.128         0.09                                             Kerosene       4.648         0.11                                             Hexadecane     5.866         0.13                                             Dodecane       4.654         0.13                                             Dodecyl acetate                                                                              5.787         0.15                                             Dibutyl phosphate                                                                            4.615         0.18                                             Oleyl alcohol  5.114         0.28                                             Trioctylamine  3.785         0.31                                             Undecyl alcohol                                                                              4.528         0.40                                             Ethyl acetate  4.550         0.41                                             Ethyl butyrate 4.665         0.42                                             Dexyl alcohol  3.890         0.42                                             Octanol        4.358         0.45                                             Nonyl alcohol  3.470         0.55                                             2-ethyl-1-hexanol                                                                            3.308         0.77                                             3-methylcyclohexanol                                                                         2.110         1.26                                             Cyclohexanone  2.702         1.66                                             Tributyl Phosphate                                                                           1.657         2.38                                             ______________________________________                                    

                  TABLE 6                                                         ______________________________________                                        Distribution Coefficients of Mixed Solvents                                                        Distribution                                                                            Percent                                        Solvent Mix          Coefficients                                                                            Increase                                       ______________________________________                                        Oleyl Alcohol (10 cc)                                                                              0.17                                                     Oleyl Alcohol (10 cc) + Cyc (1 cc)                                                                 0.31       72                                            Oleyl Alcohol (10 cc) + TBP (1 cc)                                                                 0.29       61                                            Oleyl Alcohol (10 cc) + Cyc (2 cc)                                                                 0.45      150                                            Oleyl Alcohol (10 cc) + TBP (2 cc)                                                                 0.42      133                                            Oleyl Alcohol (10 cc) + Cyc (3 cc)                                                                 0.36      100                                            Oleyl Alcohol (10 cc) + TBP (3 cc)                                                                 0.42      133                                            Oleyl Alcohol (10 cc) + Cyc (4 cc)                                                                 0.35       94                                            Oleyl Alcohol (10 cc) + TBP (4 cc)                                                                 0.40      122                                            Oleyl Alcohol (10 cc) + Cyc (6 cc)                                                                 0.52      188                                            Oleyl Alcohol (10 cc) + TBP (6 cc)                                                                 0.65      261                                            Oleyl Alcohol (10 cc) + Cyc (7 cc)                                                                 0.69      283                                            Oleyl Alcohol (10 cc) + TBP (7 cc)                                                                 0.74      311                                            ______________________________________                                    

                  TABLE 7                                                         ______________________________________                                        CO Consumption by ERI2 in the Presence of Surfactants                                                  With                                                                   Control*                                                                             Surfactant                                           ______________________________________                                        DNAP (0.1%, v/v)    100      0                                                Nondiet P-40 (0.1%, v/v)                                                                          100      0                                                Tergitol NP-10 (0.1%, v/v)                                                                        100      0                                                Tergitol Min Foam 1X (0.1%, v/v)                                                                  100      0                                                Tergitol TMN-10 (0.1%, v/v)                                                                       100      0                                                Triton X-15 (0.1%, v/v)                                                                           100      0                                                Triton X-100 (0.1%, v/v)                                                                          100      0                                                Triton X-114 (0.1%, v/v)                                                                          100      0                                                Triton N-101 (0.1%, v/v)                                                                          100      5.83                                             Triton X-405 (0.1%, v/v)                                                                          100      7.82                                             Tergitol 8 (0.1%, v/v)                                                                            100      12.15                                            Triton N-42 (0.1%, v/v)                                                                           100      42.90                                            Witconol NS-500K (0.01%, w/v)                                                                     100      79.08                                            Tween 85 (0.1%, v/v)                                                                              100      82.16                                            Witconol H-33 (0.1%, v/v)                                                                         100      90.12                                            Witconol 6903 (0.1%, v/v)                                                                         100      92.39                                            Tween 80 (0.1%, v/v)                                                                              100      97.15                                            Arlacel 83 (0.1%, v/v)                                                                            100      97.43                                            Span 80 (0.1%, v/v) 100      99.12                                            Tyloxapol (0.1%, v/v)                                                                             100      104.86                                           Witconol 5906 (0.1%, v/v)                                                                         100      108.42                                           Span 85 (0.1%, v/v) 100      124.85                                           W-1 (0.001%, w/v)                                                             First time          100      105.89                                           Second time regas   100      0                                                Brij 96 (0.004%, w/v)                                                         First time          100      107.98                                           Second time         100      0                                                ______________________________________                                    

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Kerby, R. and J. G. Zeikus. "Growth of Clostridium thermoaceticum on H₂/CO₂ or CO as Energy Source," Curr. Microbiol. (1983) 8:27-30.

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What is claimed is:
 1. A process for producing a fermentation productselected from the group consisting of acetic acid and salts thereofcomprising the steps of (a) continuously fermenting a gas streamcomprising CO anaerobically with Clostridium ljungdahlii ATCC No. 55380in an aqueous nutrient media in a bioreactor to produce a brothcontaining said product; (b) continuously removing at least a portion ofthe said broth from said bioreactor; and (c) recovering said productfrom said broth removed from said bioreactor.
 2. The process accordingto claim 1 wherein said gas stream further comprises CO₂ and H₂.
 3. Theprocess according to claim 1 wherein said nutrient medium compriseswater, minerals, vitamins and trace metals.
 4. The process according toclaim 1 wherein said gas stream is generated by industrial processesselected from the group consisting of carbon black production, ammoniaproduction, methanol production, petroleum refining and cokemanufacture.
 5. The process according to claim 1 wherein said bioreactoris selected from the group consisting of a continuously stirredbioreactor, an immobilized microbial cell bioreactor, a trickle bedbioreactor, a bubble column bioreactor, and an air lift fermenterbioreactor.
 6. The process according to claim 1 further comprisingmaintaining said bioreactor at a pressure of greater than oneatmosphere.
 7. The process according to claim 1 further comprisingintroducing a surfactant into said bioreactor.
 8. The process accordingto claim 1 wherein said recovering step comprises separating saidproduct from said bacteria by passing said removed product-containingbroth through a separation unit; returning the bacteria to thebioreactor to maintain a high bacteria concentration, thereby producinga bacteria-free, product-containing stream.
 9. The process according toclaim 8 wherein said separating comprises centrifugation orultracentrifugation.
 10. The process according to claim 8 wherein saidseparating comprises filtering said broth through a hollow fibermembrane.
 11. The process according to claim 1 wherein said recoveringstep comprises contacting said removed broth containing acetic acid witha water-immiscible solvent having a high affinity for acetic acid in acounterflow mixing vessel.
 12. The process according to claim 1 whereinsaid product is acetic acid and said recovering step comprises solventextraction and distillation.
 13. The process according to claim 1further comprising the step of separating said bacterial strain from theremoved broth prior to recovering acetic acid therefrom.
 14. The processaccording to claim 13 further comprising the step of adding saidbacterial strain from said removed broth to the bioreactor to maintain ahigh bacteria concentration therein.
 15. The process according to claim14 wherein said recovering step includes the steps of extracting aceticacid using at least one solvent miscible with the said acetic acid andsubstantially immiscible with water by mixing the removed broth with thesolvent to form an aqueous phase and a solvent phase containing saidacetic acid and separating the product from the solvent phase bydistilling the solvent phase.
 16. The process according to claim 15wherein said solvent phase contains water, and said process furthercomprises separating said water from said solvent phase and returningthe water to the bioreactor.
 17. The process according to claim 10further comprising the steps of recycling said aqueous phase back tosaid bioreactor and recycling said acetic acid-free solvent phase to beused as solvent.
 18. The process according to claim 1 comprisingoperating said bioreactor in a pH range lower than 5.5.
 19. A processfor producing a fermentation product selected from the group consistingof acetic acid and salts thereof comprising the steps of (a)continuously fermenting a gas stream comprising CO₂ and H₂ anaerobicallywith Clostridium ljungdahlii ATCC No. 55380 in an aqueous nutrient mediain a bioreactor to produce a broth containing said product; (b)continuously removing at least a portion of the said broth from saidbioreactor; and (c) recovering said product from said broth removed fromsaid bioreactor.
 20. The process according to claim 19 wherein saidnutrient medium comprises water, minerals, vitamins and trace metals.21. The process according to claim 19 wherein said gas stream isgenerated by industrial processes selected from the group consisting ofcarbon black production, ammonia production, or methanol production,petroleum refining and coke manufacture.
 22. The process according toclaim 19 wherein said bioreactor is selected from the group consistingof a continuously stirred bioreactor, an immobilized microbial cellbioreactor, a trickle bed bioreactor, a bubble column bioreactor, and anair lift fermenter bioreactor.
 23. The process according to claim 19further comprising maintaining said bioreactor at a pressure of greaterthan one atmosphere.
 24. The process according to claim 19 furthercomprising introducing a surfactant into said bioreactor.
 25. Theprocess according to claim 19 wherein said recovering step comprisesseparating said product from said bacteria by passing said removedproduct-containing broth through a separation unit; returning thebacteria to the bioreactor to maintain a high bacteria concentration,thereby producing a bacteria-free, product-containing stream.
 26. Theprocess according to claim 25 wherein said separating comprisescentrifugation or ultracentrifugation.
 27. The process according toclaim 25 wherein said separating comprises filtering said broth througha hollow fiber membrane.
 28. The process according to claim 19 whereinsaid recovering step comprises contacting said removed broth containingacetic acid with a water-immiscible solvent having a high affinity foracetic acid in a counterflow mixing vessel.
 29. The process according toclaim 19 wherein said product is acetic acid and said recovering stepcomprises solvent extraction and distillation.
 30. The process accordingto claim 19 further comprising the step of separating said bacterialstrain from the removed broth prior to recovering acetic acid therefrom.31. The process according to claim 30 further comprising the step ofadding said bacterial strain from said removed broth to the bioreactorto maintain a high bacteria concentration therein.
 32. The processaccording to claim 31 wherein said recovering step includes the steps ofextracting acetic acid using at least one solvent miscible with the saidacetic acid and substantially immiscible with water by mixing theremoved broth with the solvent to form an aqueous phase and a solventphase containing said acetic acid and separating the product from thesolvent phase by distilling the solvent phase.
 33. The process accordingto claim 32 wherein said solvent phase contains water, and said processfurther comprises separating said water from said solvent phase andreturning the water to the bioreactor.
 34. The process according toclaim 33 further comprising the steps of recycling said aqueous phaseback to said bioreactor and recycling said acetic acid-free solventphase to be used as solvent.
 35. The process according to claim 19comprising operating said bioreactor in a pH range lower than 5.5.